Generally, semiconductor chips used in electronic devices comprise a semiconductor die mounted on a carrier or a substrate. The semiconductor die is manufactured using standard semiconductor fabrication processes and has a series of bond pads that are used to make electrical contact to the integrated circuit formed therein. The die is placed on a carrier or substrate that has electrical leads formed therein to correspond to the bond pads of the die. Solder balls may be used to attach the die to the substrate. The die and the carrier may be enclosed to protect the die from the environment.
In some situations, particularly with complementary metal-oxide semiconductor (CMOS) image sensors, it is preferred that a cavity be formed over some of the circuitry. Generally, CMOS image sensors utilize light-sensitive CMOS circuitry to convert light energy into electrical energy. The light-sensitive CMOS circuitry typically comprises a photo-diode formed in a silicon substrate. As the photo-diode is exposed to light, an electrical charge is induced in the photo-diode. The photo-diode is typically coupled to a MOS switching transistor, which is used to sample the charge of the photo-diode. Colors may be determined by placing filters over the light-sensitive CMOS circuitry.
After the CMOS image sensors and related circuitry have been formed, the CMOS image sensors are packaged by placing a packaging layer over the wafer on which the CMOS image sensors are formed. Prior to placing the packaging layer over the wafer, which is typically formed of a transparent material such as glass, a spacer material, such as an epoxy, is formed and patterned on the packaging layer. The packaging layer is then aligned and affixed to the substrate to form a combined substrate. Thereafter, the combined substrate may be diced to separate the individual dies and individually package as required by the particular application requirements. An optical epoxy typically fills any voids between the packaging layer and the wafer.
This method of packaging CMOS image sensors, however, may be difficult to control. In particular, the spacer material formed on the packaging layer must be precisely aligned with the individual die. Any misalignment may cause the device to fail, thereby reducing yields and increasing costs. Accordingly, there is a need for an efficient and effective method to package a semiconductor structure having a cavity.